Measurement of torque under dynamic conditions requires measurement of the amount of twist on a rotating shaft and the calculation of torque therefrom knowing the parameters of the shaft. This type of measurement has heretofore required some form of rotating activation mechanism attached to the shaft which activates an external sensor and provides for measurement of angular deviations of the rotating shaft. These external sensors have previously comprised optoelectronic devices, transformers or capacitive mechanisms, and have required considerable and expensive external excitation and detection circuitry to extract the signal from the rotating activation devices.
U.S. Pat. No. 3,938,890 issued to Flavell on February 1976, U.S. Pat. No. 3,487,402 issued to Hillhouse on December 1969 and U.S. Pat. No. 3,940,979 issued to Ward on March 1976, illustrate examples of prior contactless, indirect torque measurement devices. Ward and Flavell teach the use of optical sensing of the angular displacement, while Hillhouse discloses the use of a capacitive measuring device.
The Ward patent discloses a light source and a detector to detect the presence of slots on the periphery of two rotating disks. Each of the rotating disks is attached to two separate points spaced a predetermined distance apart on the surface of the torsioned shaft. By measuring the relative position of the slots on the two disks with the detected light from the light source, an indication of the twist in the shaft can be obtained. However, this device requires precise alignment of the two disks and the measurement of a light level to determine the relation between the slots and the two disks. This is a disadvantage in that various factors such as contamination and misalignment can affect the intensity of the light, thereby introducing a probability of error into the twist measurement. Such disadvantages are also seen in the Flavell device, which utilizes two polarizing disks with light sources shining therethrough.
In both the Ward and the Flavell devices, the light source is disposed on one side of the disks and the detector is disposed on the other side thereof. Therefore, the light from the light source passing through the two disks is modulated by the slots in Ward and the polarizing axis in Flavell. It is this modulation of the light that is detected to provide an indication to an external device of the actual twist. However, the light source, and the modulation thereof, is susceptible to many external parameters which can introduce error into the measurement.
In view of the above disadvantages, there exists a need for a contactless, indirect torque measurement device that is not subject to the inherent limitations of previously developed devices, etc.